The invention relates to an apparatus for conveying a copy sheet for an electrophotographic copying machine, and more particularly, to such an apparatus which may be utilized to convey a copy sheet carrying an unfixed toner image thereon to a fixing unit.
In an electrophotographic copying machine, a conveying apparatus is used to transport a copy sheet onto which a toner image has been transferred at a transfer station, to a fixing unit. In such a conveying apparatus, it is desirable that the conveying operation can be achieved without utilizing a first surface of the copy sheet which carries the unfixed toner image, while utilizing only the second surface thereof in order to prevent any damage caused to the toner image. However, to convey a copy sheet which is very light in weight in an accurate manner by means of rollers and belt, the copy sheet must be attracted to these rollers or belt with a frictional force of a suitable magnitude.
To this end, a variety of means are proposed in the prior art. By way of example, FIG. 1 shows a conveying roller 4 having a peripheral surface which is charged to the opposite polarity from that of a copy sheet 1 carrying a toner image 2 on its surface so as to produce an attraction therebetween. Alternatively, FIG. 2 illustrates a suction unit 4b which attracts the copy sheet 1 against the upper surface of a conveyor belt 4a. In FIGS. 1 and 2, numeral 3 indicates a heating and fixing unit. It will be appreciated that the use of the electrostatic interaction in the arrangement of FIG. 1 is adversely influenced by the humidity and requires an expensive power supply unit, resulting in an increased cost. The use of the suction unit in the arrangement of FIG. 2 results in an increased overall size and a complex construction, disadvantageously causing a substantial increase in the cost.
To avoid these disadvantages, there is proposed an arrangement as illustrated in FIGS. 3 and 4 where a roller 5a is disposed to bear against the marginal portion of the front surface of the copy sheet in a region where no image is produced, thereby holding the copy sheet 1 between it and a drive roller 5b which is disposed on the opposite side thereof. However, this arrangement causes an instability in the direction in which the copy sheet 1 is being conveyed. In particular, where a copy sheet of half size is used, the roller 5a can be located on only one lateral side, giving rise to the likelihood that the copy sheet may be skewed. In this instance, high mechanical accuracy is required of the combination of rollers 5a, 5b which hold the copy sheet 1 therebetween, resulting in a troublesome operation.
As a further alternative, FIG. 5 illustrates the use of a roller 6 in the form of a spur wheel or having acicular projections so that the area of its contact with the surface of the copy sheet 1 may be minimized. A plurality of such rollers may be used in combination with a drive roller 5c. However, the tip ends of these rollers scar or damage the surface of the copy sheet 1 and carry the toner over to non-image areas, thus marring the copy sheet.
Finally, FIG. 6 illustrates the use of a disc-shaped roller 7 having a reduced thickness and bearing against the front surface of the copy sheet 1, in combination with a cleaning member 9 which operates to scrape any toner 8 off the peripheral surface of the roller 7. With this arrangement, when it is used to convey a number of copy sheets, an accumulation of the toner 8 removed is formed on the cleaning member 9, eventually resulting in marring the roller 7 or falling of the toner onto the copy sheet 1 directly to mar the copy sheet 1.